re PR tree-optimization/88775 (Optimize std::string assignment)

re PR tree-optimization/88775 (Optimize std::string assignment)

	PR tree-optimization/88775
	* match.pd (cmp (convert1?@2 addr@0) (convert2? addr@1)): Optimize
	equal == 0 equality pointer comparisons some more if compared in
	integral types and either one points to an automatic var and the
	other to a global, or we can prove at least one points to the middle
	or both point to start or both point to end.

	* gcc.dg/tree-ssa/pr88775-1.c: New test.
	* gcc.dg/tree-ssa/pr88775-2.c: New test.

From-SVN: r267931